The Carrier – based PWM method to Reduce Common-mode Voltage for Three – level T – Type Neutral Point Clamp Inverter *

Tuyen D. Nguyen *, Phan Quoc Dzung *, Dao Ngoc Dat *, Nguyen Huu Nhan** Faculty of Electrical and Electronics Engineering, Hochiminh City University of Technology, Vietnam [email protected] ** School of Electrical Engineering, University of Ulsan, Korea

Abstract— Common-mode voltage (CMV) is responsible for overvoltage stress to the winding insulation and bearing damage of an AC motor. High dv/dt of CMV causes leakage currents, which create noise problems to the equipments installed near the converter. This paper proposes modulation strategy for threelevel T-type NPC that substantially eliminates CMV. The principles of selecting suitable triangle carrier signals for the Ttype NPC is described. The proposed method can mitigate the peak value to 50% as compared to the conventional sinusoidal pulse width modulation method. The proposed modulation can be easily implemented in software without any additional hardware modifications. Both simulation and experimental results are shown to demonstrate that the new carrier-based PWM method can generate good performance of the output waveforms and provide CMV reduction. Keywords—Sinusoidal PWM, Carrier-based PWM, Commonmode voltage, three-level inverter, T-type NPC.

or electromagnetic interference filters in order to mitigate the CMV [6], [7]. The increasing of converter volume, weight and cost is the drawback of this method. The second solution is to investigate the modulation technique. Using suitable PWM, mitigating the CMV is achievable within the power converter. Several studies have been proposed to reduce the CMV for NPC topology by applying the proper modulation techniques. Nevertheless, these methods focus on SVPWM method. However, the approach of SVPWM has some disadvantages [8], [9]. It needs complex calculations and tables to synthesize reference output voltage. This paper focuses on the carrier-based PWM method for T-NPC to eliminate the CMV. In the carrier-based PWM method, the high frequency triangular carrier is compared to the modulation signals to create the gating pulses for the switches in the power circuit. The proposed method is simple to implement and does not require any additional hardware. II. THE CMV ANALYSIS AND THE PROPOSED CARRIERBASED PWM METHOD WITH CMV REDUCTION

I. INTRODUCTION In last decade, there is growing interest in multilevel converter, because of many possibilities of expanding areas of power electronics use such as: military, e-vehicle, renewable energy, etc. The multilevel inverter has some advantages such as: reduced voltage constraints on the switches, as well as lowered total harmonic distortion (THD) of the output voltages. The main drawback of multilevel converters is number of switches, which is growing when number of levels is increasing [1] – [3]. The multilevel inverters are classified in to three groups as: the neutral – point – clamped (NPC), floating point capacitor, cascade H – bridge, while NPC is the most widely used. Recently the T-type NPC (T-NPC) is proposed to alternate the traditional NPC in order to improve the efficiency [4], [5]. Common-mode voltage (CMV) due to the high-speed pulse width modulation (PWM) in power converters introduces many problems within an electrical system. It is the main source of early motor winding failure, bearing deterioration and wide-band electromagnetic interference. It is important to reduce the CMV within the power converter. Two main solutions that are used to reduce CMV are presented in previous researches. The first solution is based on the use of additional hardware such as a passive filter, active-cancellers

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A. The T-NPC Inverter Topology The T-NPC inverter topology is illustrated in Fig. 1. As shown in this figure, each leg of the TNPC consists of four power devices: the T1 and T4 is connected positive and negative pole of DC power supply, respectively and one

Fig. 1. The three-level T-NPC Inverter topology.

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Switching State SA, SB, SC +1 0 -1

TABLE 1: THE SWITCHING STATES OF THE T-NPC Switching Status Output Voltage VAg S1 S2 S3 S4 ON ON OFF OFF V0/2 OFF ON ON OFF 0 OFF OFF ON ON -V0/2

bidirectional switch (T2 and T3) which connect the middle point of dc – link to the load.The T-NPC topology is simpler than the conventional NPC. The T-NPC provides some advantages that are not available in conventional NPC such as: low conduction losses, fewer numbers of power switches and small size. The switching state of the power switches and the output voltage of the T-NPC are shown in Table I. There are 3 levels of phase to common point voltage that can be generated on each phase leg: +V02 , 0, -V0/2, where V0 is the dc-link voltage. This corresponds to three possible states symbolized as • 1, 0, and + 1. The SVM is a pulse width modulation strategy that uses the concept of space vectors to compute the duty cycles of the switches. To implement this modulation strategy to the TNPC, the output phase voltages generated by the switching states of the T-NPC have to be converted into space vectors using the following transformation:

Vdq =

2 V + Vb e 3 a

2π j 3

+ Vb e

4π j 3

(1)

Fig. 2 shows the space vector diagram of the NPC in the dq plane that is formed by the voltage space vectors which includes four groups: zero voltage vectors, small voltage vectors, medium voltage vectors, and large voltage vectors. The space vector modulation (SVM) method needs many calculations and tables to make the switching pattern according to the output voltage sectors. Their implementation is unintuitive because the gating signals are made from the duty cycles of the effective vectors which are calculated by the equations. Therefore, the carrier-based PWM method is proposed for T-NPC in order to simplify the modulation algorithm.

B. Common-Mode Voltage Analysis The CMV of the T-NPC is defined as the voltage between the load neutral point “n” and the mid-point of dc-link voltage “g”, as shown in Fig. 1. It is assumed that the three-phase RL load is a balanced load. The CMV can be derived from the following equations:

v Ag = vng + R A i A + L A

di A dt

v Bg = v ng + R B iB + L B

diB dt

vCg = v ng + RC iC + LC

(2)

diC dt

where vAg, vBg, and vCg are three output phase voltages with respect to the ground point “g”, and iA, iB, and iC are three output phase currents. Since the three-phase load is balanced, the CMV can be written as:

v ng =

(

1 v Ag + v Bg + vCg 3

)

(3)

The CMV depends on the switching states T-NPC. Using the switching functions, the relationship is determined as follows:

Vng =

V0 (S + Sb + Sc ) 6 a

(4)

From (4) and Table 1, the CMV has seven values: 0, ±V0/6, ±V0/3, ±V0/2, which is described in detail in table 2. Regardless of the AC source, the CMV is determined by the output voltages which depend only on the switching states of T-NPC. Fig. 3 shows the basic principle of sinusoidal pulse width modulation (SPWM) with double triangle carrier signals. Where VA, VB, VC are represented three phase output voltage. The upper carrier signal (Vcarrier1) is used to generate the gate signal for switch S1, S3 and the lower carrier signal is used to generate the gate signal for the switch S2, S4. It can be seen that the peak value of CMV is -V0/3, which is caused by the switching state 0-1-1. Based on Table 2 and Fig. 3, a new carrier-based PWM is proposed in this paper to reduce the peak of CMV, which is described in next section. TABLE 2. THE CMV ACCORDING TO THE SWITCHING STATES Switching state 000,-101,10-1,01-1, 0-11,1-10,-110 111 -1-1-1 110, 101, 011 -1-10, -10-1, 0-1-1 1-1-1,11-1,-11-1, -100,0-10,00-1 11-1,1-11,-111, 100,010,001

Fig. 2. The space vector diagram of three-level T-NPC topology.

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CMV 0 V0/2 -V0/2 V0/3 -V0/3 -V0/6 V0/6

generated gating pulse the three-level T-NPC. As shown in Fig. 4, Vcarrier1A and Vcarrier2A are used to generate gating pulse for four switches of leg A. Similarly, Vcarrier1B, Vcarrier2B, Vcarrier1C and Vcarrier2C are used to generate gating pulse for four switches of leg B and leg C, respectively.

1 vA vcarrier1

It is assumed that the switching state of phase A is 0, and the switching state of phase B and C are -1, which causes the peak value of CMV at V0/3. Therefore, the relationship between the reference output voltage and the carrier signals are given in 8.

0 vB vC

0 < VA < Vcarrier1A VB < Vcarrier 2 B < 0

vcarrier2 -1 SA

1

1

1

0

1

1

1

SB

0

0 -1

-1

-1

0

0

SC

0

-1 -1

-1

-1

-1

0

(8)

VC < Vcarrier 2C < 0 It is noted that VA + VB + VC = 0

(9)

From (8) and (9), we have:

Vcarrier1A + Vcarrier1B + Vcarrier1C > 2

Fig. 3. The reference output voltage, triangle carrier signals and switching states of the conventional SPWM.

C. The Proposed carrier-based PWM Method with CMV reduction From Table 2, by employing 19 switching state (000, -101, 10-1, 01-1, 0-11, 1-10, -110, 1-1-1, 11-1, -11-1, -100,0-10,001, 11-1, 1-11, -111, 100,010,001) to synthesis the reference output voltage, the CMV is limited at V0/6. It is assumed that the normalized three-phase outputs are given by: VA = m sin (ωt ) (5)

2π · § VB = m sin ¨ ωt − ¸ 3 ¹ ©

(6)

2π · § VC = m sin ¨ ωt + 3 ¸¹ ©

(7)

where m is the modulation index, ω is the input angular frequency. Since the three phase output voltages are balanced, there are two possible conditions for the six difference situations. In the first condition, two input phase voltages are positive and one phase voltage is negative. In the second condition, two input phase voltages are negative and one phase voltage is positive. Without missing the generality of the analysis, we assume that the output voltage of phase A is positive and output voltage of phase B and C are negative as shown in Fig. 3. Therefore, the possible switching states for phase A are 1 or 0, and the possible switching states for phase B and C are 0 or -1. As shown in Fig. 3, the switching state 0-1-1 makes the CMV at –V0/3. Thus, to reduce the peak of CMV, it has to avoid using this switching state to synthesis the reference output voltages. In the conventional SPWM method, two carrier signals are used. In this paper, the new carrier-based PWM is proposed, where six carrier signals are used to

(10)

Fig. 4 shows the three upper carrier signals for three legs A, B, C with the conventional SPWM method. It can be seen that, the sum of three carrier signals is larger than 2. Therefore, the peak of CMV with the value V0/3 is obtained. To avoid the inequality (10) happened, the upper carrier signal for phase B and phase C are chosen with the displaced by 1/3 of a switching period as shown in Fig. We can see that the sum of three carrier signals is smaller than 2. So, the maximum of CMV is V0/6. The carrier signals for three legs and the switching state are illustrated in Fig. 5. It can be seen that the switching state 0-1-1 does not appear. III. SIMULATION AND EXPERIMENTAL RESULTS D. Simulation results Simulations are carried out for a three-phase RL load using PSIM 9.0 software. The simulation parameters are as follows: - Dc-link voltage V0=320 V. - Three-phase RL load: R=20 ȍ, L = 5 mH. - Modulation index m =0.85. - Output frequency: fout = 50Hz. - The PWM frequency is 10 kHz.

(a)

(b)

Fig. 4. Three triangle carrier signals of (a) the conventional SPWM (b) the proposed SPWM.

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VP6 480

360

240

120

0

-120

-240

-360

-480 VP5

300

200

100

0

-100

-200

-300

0.1

0.104

0.108

0.112

0.116

0.12 T ime (s)

0.124

0.128

0.132

0.136

0.14

Fig. 6. The line-to-line and phase output voltage with the conventional SPWM. I(RL1)

I(RL2)

I(RL3)

15

10

5

0

-5

-10

-15

VP3 200

100

0

-100

-200

0.1

0.104

0.108

0.112

0.116

0.12 Time (s)

0.124

0.128

0.132

0.136

0.14

Fig. 7. The three phase output currents and CMV waveforms with the conventional SPWM method. VP5 480

360

240

120

0

-120

-240

-360

-480

300

VP4

200

100

0

-100

-200

-300

0.1

0.104

0.108

0.112

0.116

0.12 T im e (s)

0.124

0.128

0.132

0.136

0.14

Fig. 8. The line-to-line and phase output voltage with the proposed SPWM. Fig. 5. The reference output voltage, triangle carrier signals and switching states of the proposed SPWM.

I(RL1)

I(RL2)

I(RL3)

20

10

Fig. 6 show the simulation results of the line-to-line and phase output voltage at m=0.8.5 for the conventional method. The three-phase output current and the CMV are shown in Fig. 7. The peak value of the CMV is achieved at 106.7 V, which corresponds to Vo/3. Figs. 8 and 9 show the output voltage, output current and the CMV of the T-NPC when the new SVM method is utilized. As shown, the peak value of CMV decreases from 106.7 V to 53.4 V, which corresponds to 1/6 of dc-link voltage. According to the fast Fourier transform (FFT) of the CMV, which are shown in Figs. 10 and 11, the new SPWM method generates a smaller harmonic component compared to that in the conventional SPWM method.

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0

-10

-20 VP3 200

100

0

-100

-200

0.1

0.104

0.108

0.112

0.116

0.12 T i me (s)

0.124

0.128

0.132

0.136

0.14

Fig. 9. The three phase output currents and CMV waveforms with the conventional SPWM method.

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Fig. 13. The line-to-line and phase output voltage with the conventional SPWM. Fig. 10. The CMV’s FFT with the conventional SPWM method (50V/div, 10 KHz/div).

Fig. 14. The three phase output currents and CMV waveforms with the conventional SPWM method.

Fig. 11. The CMV’s FFT with the proposed SPWM method (50V/div, 10 KHz/div).

E. Experimental Results In order to validate the proposed theories and simulations, an experimental setup is designed in the laboratory. The experimental prototype is shown in Fig. 12. The half-bridge IGBT Modules SEMiX202GB128Ds (1200V) were selected for switch pairs: (IGBT1A-C,IGBT4A-C), while the bidirection IGBT modules SK60GM123 (1200V) were chosen for the switch pairs (IGBT2A-C, IGBT3A-C). The prototype is controlled by a high performance, floating point digital signal processor (DSP) 90MHz TMS320F28069 from Texas Instruments. The parameters used in the experiment are the same as in the simulation. The PWM operates at 10 kHz.

Fig. 15. The line-to-line and phase output voltage with the proposed SPWM.

Fig. 16. The three phase output currents and CMV waveforms with the conventional SPWM method.

Fig. 12. The laboratory T-NPC prototype.

Figs. 13 and 14 show the phase, line-to-line output voltage, three-phase output current and CMV with the conventional SPWM method, respectively, at an output frequency of 50 Hz and a voltage transfer ratio of 0.85. Figs. 15 and 16 show the phase, line-to-line output voltage, three-phase output current and CMV with the proposed SPWM method, respectively, at an output frequency of 50 Hz and a voltage transfer ratio of 0.85. Compared to the conventional SPWM method, the new SPWM method significantly reduces the CMV peak value by 50%. Furthermore, the RMS of the CMV shows that the CMV of the new SPWM method smaller than in the standard SVM

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method. The RMS value of the CMV with the conventional method is 57V, while RMS value of the CMV with the conventional method is 27V. These experimental results confirm the effectiveness of the new SPWM method for T-NPC, which is useful in reducing the CMV. IV. CONCLUSIONS A new SPWVM method for three-level T-NPC, used to reduce the CMV, is proposed in this paper. The proposed method achieves a peak value of CMV that is reduced to 50% as compared to conventional SPWM. Furthermore, the proposed method provides some advantages such as simple implementation as compared to those of the SVPWM method, the sinusoidal three phase output currents. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed method.

[2]

[3]

[4]

[5]

[6]

[7]

ACKNOWLEDGMENT This work was supported by Vietnam National University HCM City, PERLAB (HCMUT) under grant number B201220-04TD and (in part) by the NRF of Korea Grant funded by the Korea Government (No. 2010-0025483). REFERENCES [1]

Rodriguez, J.; Jih-Sheng Lai; Fang Zheng Peng, "Multilevel inverters: a survey of topologies, controls, and applications," Industrial Electronics, IEEE Transactions on , vol.49, no.4, pp.724,738, Aug 2002.

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Jih-Sheng Lai; Fang Zheng Peng, "Multilevel converters-a new breed of power converters," Industry Applications, IEEE Transactions on , vol.32, no.3, pp.509,517, May/Jun 1996. Rodriguez, J.; Bernet, S.; Bin Wu; Pontt, J.O.; Kouro, S., "Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives," Industrial Electronics, IEEE Transactions on , vol.54, no.6, pp.2930,2945, Dec. 2007. Ui-Min Choi; Hyun-Hee Lee; Kyo-Beum Lee, "Simple Neutral-Point Voltage Control for Three-Level Inverters Using a Discontinuous Pulse Width Modulation," Energy Conversion, IEEE Transactions on , vol.28, no.2, pp.434,443, June 2013. Schweizer, M.; Kolar, J.W., "High efficiency drive system with 3-level T-type inverter," Power Electronics and Applications (EPE 2011), Proceedings of the 2011-14th European Conference on , vol., no., pp.1,10, Aug. 30 2011-Sept. 1 2011. H. Akagi, H. Hasegawa and T. Doumoto, "Design and performance of a passive EMI filter for use with a voltage-source PWM inverter having sinusoidal output voltage and zero common-mode voltage," IEEE Trans. Power Electro.,, vol.19, no.4, pp. 1069- 1076, July 2004. H. Akagi and T. Doumoto, "An approach to eliminating high-frequency shaft voltage and ground leakage current from an inverter-driven motor," IEEE Trans. Ind. Appl.,, vol.40, no.4, pp. 1162- 1169, July-Aug. 2004. Bharatiraja, C.; Raghu, S.; Palanisamy, R., "A new space vector pulse width modulation for reduction of common mode voltage in three level neutral point diode clamped multilevel inverter," Advances in Engineering, Science and Management (ICAESM), 2012 International Conference on , vol., no., pp.694,699, 30-31 March 2012. Lizhong Long; Yonggao Zhang; Guangjian Kuang, "A modified space vector modulation scheme to reduce common mode voltage for cascaded NPC/H-bridge inverter," Power Electronics and Motion Control Conference (IPEMC), 2012 7th International , vol.3, no., pp.1837,1841, 2-5 June 2012.

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